Optical measurements are frequently implemented to determine physiological parameters of a living being, in particular blood-related parameters. Devices, called pulse oximeters, are commonly used to determine an oxygen saturation of the haemoglobin in the blood. These measurements are based on the absorption of light by the haemoglobin, the latter varying between oxyhaemoglobin and deoxyhaemoglobin. Commercial devices are widely used, these devices being based on a measurement of the light transmitted by a sufficiently thin member, in an infrared spectral band and in a red spectral band. The examined member may in particular be the end of a finger or the lobe of an ear. Measurements of the transmitted light, in each of the spectral bands, allow concentrations of oxyhaemoglobin and deoxyhaemoglobin in the blood to be estimated, from which the oxygen saturation of the blood is estimated. These measurements also allow a pulsatile blood flow to be detected and a cardiac frequency to be deduced therefrom. Most commercial oximeters also allow the cardiac frequency to be estimated from measurements carried out in one or other spectral band.
European patent EP2355693 for example describes a device including a first light source emitting at a red wavelength and a second light source emitting at an infrared wavelength. A photodetector is configured to detect light radiation emanating from a finger illuminated by one of the light sources. The infrared light source is in particular used to detect the presence of the finger against the device, subsequent to which the red light source is activated, so as to allow parameters such as cardiac frequency or pulse oximetry to be determined.
U.S. Pat. No. 9,042,971 describes a portable actigraphy device allowing a cardiac frequency to be determined optically via detection of light radiation backscattered by a finger under the effect of an illumination. This device allows measurements to be carried out in a back-scatter configuration, also called the reflectance configuration, the light sources being placed adjacent to the photodetector.
The inventors have observed that such devices may make errors when measuring cardiac frequency, in particular when they are worn by a moving person. The invention proposes to solve this problem, and hence allow more reliable measurements of cardiac frequency to be obtained.